The present invention relates to the field of hot melt ink jet inks for use with impulse ink jet (drop on demand) type printers. More particularly, the present application relates to such ink jet inks which utilize solid pigments rather than soluble dyes as their colorants.
Originally, printing with hot melt type ink was suggested in connection with electrostatic printing apparatuses. In U.S. Pat. No. 3,653,932 (Berry et al), entitled "Electrostatic Printing Composition Comprising Didodecyl Sebacate", an electrostatic printing process is disclosed using an ink comprised of one or more specified di-esters. In the Berry process, ink is heated into a fluid phase and is caused to form a convex meniscus at the nozzle tip by hydrostatic pressure. This pressure causes the end of the ink to intrude into an electrostatic field. Ink is then electrostatically drawn into a single file stream of droplets which traverses the span between the tip of the nozzle and the carrier. The preferred inks for use in this process are di-esters of sebacic acid which have been esterified with alcohols of paraffins having 12 or less carbon atoms in their chains. Each of the preferred inks is disclosed as having a melting point "which does not exceed about 51.degree. C. and a freezing point which is not below 30.degree. C.", to ensure that the ink will be in a liquid phase at the operating temperature at the exemplary reservoir, namely, about 56.degree..+-.3.degree. C., and that it "will be solid at generally encountered room temperatures to minimize its flow from the carrier". Berry et al discloses that such di-esters "provide excellent vehicles for pigments and dyes for producing high quality electrostatic printing" and further discloses a number of examples using methyl violet toner, C.I. solvent yellow, C.I. solvent black. The methyl violet toner of the examples comprises about 40% methyl violet tanate in a transparent yellow mineral oil carrier.
U.S. Pat. No. 3,715,219 (Kurz et al) discloses a similiar electrostatic printing process using an ink composition comprising about 3% of a dye such as C.I. solvent black or C.I. solvent yellow. The remainder of the Kurz et al ink is a vehicle comprised of at least one alcohol of the paraffin series which has the general formulus CH.sub.3 (CH.sub.2).sub.n CH.sub.2 OH, in which n is an integer between 12 and 26. According to Kurz, the preferred hot melt type inks have melting points which do not exceed about 61.degree. C. and freezing points which are not below about 30.degree. C. During the process, ink in a fluid phase is supplied at a constant flow rate from a source represented by supply tube 10 to reservoir 11 which terminates at a nozzle 12 with a capillary bore. The chamber 11 of the exemplary apparatus is maintained at a temperature in the range of about 62.degree. C. and 82.degree. C. during the disclosed process. Viscosity of the disclosed inks is said to be within a range having an upper limit of 50 centipoises at operating temperatures, the exemplary inks in the Kurz et al references having viscosities of between 4.0-5.9 at 80.degree. C.
More recently, impulse ink jet printing processes using hot melt inks have been disclosed. Impulse ink jet printing differs from electrostatic printing in that individual droplets are discharged from a printing head in response to relatively low pressures which are typically achieved using piezoelectric head elements. Unlike electrostatic printing processes, such drop on demand processes produce much larger droplets which decelerate, not accelerate as they move toward the carrier.
One recent hot melt impulse ink jet ink is disclosed in U.S. Pat. No. 4,390,369 (Merritt et al), entitled "Natural Wax-Containing Ink Jet Inks". This patent discloses the use of a number of natural waxes, such as Japan wax, candelilla wax, carnauba wax, etc. These waxes may be used at percentages of from 0.5 to 97.0% by weight either as the basic fluid vehicle of the ink or as an additive to other fluidic vehicles such as fatty acids, more particularly oleic acid or oleic acid with benzyl ether. Merritt et al disclose a number of such inks having viscosities of 6.7 to 15.7 at 165.degree. F. (about 74.degree. C.). Merritt et al disclose that a "coloring agent or a dye such as an oil or solvent soluble dye is usually added to the composition for visability". The disclosed colorants include HD Victoria Blue, Calco nigrosine base, Acetosol Yellow, RLSN, and oil black BN.
In related application Ser. No. 394,153, filed Jul. 1, 1982 (Lin et al) a number of hot melt impulse ink jet inks are disclosed which are described as being solid or semi-solid at ambient temperature. The preferred ink is disclosed as comprising stearic acid in an approximate weight range of 50-99%. As explained in that application, commercially available stearic acids are produced from saponified and distilled animal fats usually being composed of 60% liquid and 40% solid acid, the bulk of the liquid acids being separated from the solid acids by hydraulic pressing to produce for example, single pressed, double-pressed, triple-pressed stearic acids, etc. According to the Handbook of Chemistry and Physics (49th Edition), chemically pure stearic acid is octadecanoic acid (CH.sub.3 (CH.sub.2).sub.16 CO.sub.2 H (melting point 70.1.degree. C.). In addition to stearic acid, the Lin et al application discloses that additives such as oleic acids, Typophor black, nigrosine base, benzyl ether, compounded or chemically modified waxes (including natural or other synthetic substances), a coloring agent or dye, such as oil or solvent soluble dye, etc. may be used to formulate the disclosed hot melt inks.
More recently, in the aforementioned related application entitled "High Molecular Weight, Hot Melt Impulse Ink Jet Ink", hot melt impulse ink jet inks have been disclosed comprising acid or alcohol vehicles in the C.sub.20 -C.sub.24 ; range, significant proportions of a relatively high melting point ketone, such as stearone; a solid plasticizer, such as an acrylic resin; and a coloring agent, such as a dye. Alternatively, the disclosed vehicle may comprise waxes, such as candelilla wax and/or other specified waxes which melt above 65.degree. C.
Numerous prior art references disclose a wide variety of colorants for use in other (non-impulse) printing or writing applications. For example, U.S. Pat. No. 4,238,807 (Bovio et al) discloses a non-impact printing device which selectively emits solid ink particles from a rod of solid ink which is pulsed with high voltages to cause ink particles to be eroded from the rod and ejected through a nozzle on the paper. The rod of said ink is disclosed as consisting of carbon black compressed with 5 to 20% of stearic acid as a binder. In U.S. Pat. No. 4,273,847 (Lennon et al) inks are disclosed for use in pulsed electrical printing. According to this process, ink particles, such as carbon particles are charged by conduction of current from other particles closer to a supporting sheet, detached by the electric field and then caused to transfer to the receiving paper by the force induced solely by the electrical field.
Other inks containing pigment, such as carbon black are known for other purposes. U.S. Pat. Nos. 3,421,910 (Gilson et al) and 3,330,673 (Voet et al) relate to a stencil paste and news printing inks, respectively, which use a dispersion of carbon black in mineral oil. U.S. Pat. Nos. 4,337,183 (Santiago), 4,248,746 (Greiner) and 3,353,974 (Trimble) each disclose ink for purposes other than jet printers which use a carbon pigment in a vehicle. Patents including disclosures of glycerol, ethylene glycol, diethylene glycol and the like, with colorant materials such as pigments or dyes (most of which inks are water based) are disclosed in U.S. Pat. Nos. 3,421,910 (Gilson); 3,846,141 (Ostergren et al); 4,396,429 (Matsumoto et al); 4,395,287 (Kobayashi et al); 4,409,040 (Tobayashi et al); 4,421,559 (Owatari) and 4,176,361 (Kawada et al). U.S. Pat. No. 4,443,820 discloses an ink which is solid at room temperature and liquid at an elevated temperature which contains more than ten percent of a dye dispensed in a hydrocarbon of the thermally dissoluble type. The ink is deposited on a (hectographic) master by discharging from a jet generating unit. The deposited ink is cooled hence forming an image configuration of solidified ink suitable for use in hectographic printing of a number of reproductions of an original image. The ink in its liquid form has a viscosity of 3 to 20 c.p.
Notwithstanding the above-mentioned disclosures, a need exists to provide improved impulse ink jet inks which are capable of producing copy which approaches or exceeds the copy of standard impact-type printing, such as that which may be produced from a standard office typewritter or daisy-wheel printer. Such an ink should be readily jettable, easily stored and shipped for extended periods of time without separation or degradation yet be capable of providing a sharp, dark, well defined copy when placed into use.